1. Field of the Invention
The present invention relates to methods of continuous flash butt welding and to continuous flash butt-welding machines for realization of these methods, in particular to hydraulic drives of said machines, which provide for moving the workpieces being welded in the course of welding. The invention may be employed in all fields of mechanical engineering to weld workpieces, mainly of a large cross-sectional area, and also where a high output of the welding machine is required.
2. Description of the Prior Art
In contrast to continuous resistance butt welding where current is passed through workpieces whose end faces are tightly pressed against each other, continuous flash butt welding process is characterized in that one of the workpieces being welded is translated towards the other with the welding voltage on.
When the end faces of the workpieces are brought together in such a close relation that ridges of their microirregularities touch one another, electric contacts originate at the points of contact. A high current density in these points causes the metal to rapidly heat up and become liquid. Due to overheating, the formed liquid metal bridges break down, and as the workpieces are brought closer together, contacts are formed at other points.
The process of formation and breakdown of the contacts proceeds continuously, the points of contact between the workpieces change continuously as well, which results in a uniform heating of the workpieces over the entire area of their butted end faces. This provides for a high quality of the weld joint produced by upsetting the workpieces after their end faces have been heated to the required temperature.
The uniformity of heating the workpiece end faces, representing an advantage of continuous flash butt welding over continuous resistance butt welding, makes it possible to weld workpieces of large cross-sectional area, for which purpose the continuous resistance butt welding technique has proven to be incapable.
The process of continuous flash butt welding, however, inevitably involves considerable losses of metal and of heat, stemming from an explosion-like breakdown of the bridges, accompanied by expulsion of hot metal.
Improved prior art methods of continuous flash butt welding provide for reduction of said losses by means of short-time increases in the speed of feeding one of the workpieces being welded towards the other in the course of flashing.
In particular, there is known a method of continuous flash butt welding, wherein the workpieces being welded are brought together by imparting to at least one of them translatory motion towards the other with at least one of the workpieces being set into a rectilinear oscillatory motion along the line of said translatory motion, voltage is applied to said workpieces for flashing and heating them in the course of bringing them together, and then an upset force is applied to the workpieces as disclosed in U.K. Pat. No. 1,162,073.
The above method is accomplished in a clamping device of a continuous flash butt-welding machine, said device comprising current-carrying jaws to clamp the workpiece being welded, which jaws are provided with a hydraulic drive to generate an oscillatory rectilinear motion and mounted for this motion on a column having a self-contained drive to generate a translatory motion as disclosed in U.K. Pat. No. 1,350,682.
Said machine includes a hydraulic cylinder with two chambers, intended to move one of the workpieces being welded with respect to the other.
This movement is effected with the aid of the hydraulic drive which oscillates one of the workpieces being welded and which is a means for feeding hydraulic fluid alternately in either of the chambers of the hydraulic cylinder of said machine, said means communicating with the cylinder through two hydraulic lines serving alternately one as the inlet and another as the outlet line.
The hydraulic cylinder body is fixedly coupled to the machine column, and the rods of the two pistons, either of which is accommodated in a separate chamber, thrust against the walls of a slot provided in the current-carrying jaw. The oscillatory motion of the current-carrying jaws jointly with the workpiece clamped therein is effected within the gap between a projection of one of the jaws and a recess in the column, which receives the projection. The amplitude of the oscillatory motion is restricted by the stroke of either piston and is constant for both directions.
The resulting speed of the oscillating workpiece, which is the resultant of the constant speed of the translatory motion of the column and the varying speed of the oscillatory motion of the current-carrying jaws with respect to the column, varies cyclically in the course of welding. During the half-cycle of the oscillatory motion of the workpiece when after the initial contact of the end faces the workpiece moves in the direction which coincides with that of the translatory motion of the column the resultant speed rises, thus causing the contact area to increase and to reach a value at which flashing ceases and the workpieces being welded are heated by the resistance technique.
During the next half-cycle, when the workpiece moves in the reverse direction, the contact area decreases and the flashing process recommences.
Such a combined heating (by flashing and by resistance) ensures a uniform heating over the entire area of the end faces maintained in abutting relationship and minimizes at the same time the metal and heat losses.
It is to be noted that the efficiency of the above-described method is the higher, the closer the speed of the translatory motion of the column defining the relative movement of the workpieces being welded towards each other is to the speed of flashing of the end faces of said workpieces.
The speeds being equal, the pauses, i.e. time intervals in every oscillatory motion cycle during which the workpieces being welded are out of contact, are the minimum and heating is the most intense.
When the speed of the translatory motion is less than the flashing speed, the spacing between the end faces of the workpieces being welded increases after every oscillatory motion cycle, which lengthens the pauses in current flow and shortens the heating time, thus lowering the welding productivity.
When the speed of the translatory motion exceeds the flashing speed, the spacing between the workpieces after every oscillatory motion cycle decreases, which eventually leads to dampening of the oscillations and termination of flashing. The process of heating proceeds by the resistance method and subsequently transforms into a short circuit, which drastically reduces the amount of heat liberated and impairs the uniformity of its distribution over the area of the end faces maintained in abutting relationship.
To maintain an equality between the translatory motion speed and the flashing speed in practice is, however, a very difficult task, since the flashing speed depends on the temperature of the workpieces being welded, which rises in the course of welding and in its turn depends on the area of the end faces maintained in abutting relationship, the roughness of their surfaces, random variations of the welding voltage, etc.
The above equality between speeds can be in principle maintained in the course of welding by two techniques.
The first of the techniques relies on correcting the speed of the movement of the workpieces towards each other in dependence upon the value of a variable characterizing the flashing process, e.g. on the intensity of welding current, like this is accomplished for the above-described flash butt-welding method wherein the workpieces are brought together by imparting only a translatory motion to one of them as disclosed in Japan Pat. No. 46-8567.
The second technique involves an attempt to effect a self-regulation of the spacing of the workpiece faces maintained in abutting relationship as the faces get flashed.
Realization of the two above techniques is prevented since the workpieces being welded in the course of coming closer together perform two independent motions (translatory and oscillatory), which makes it difficult or as in case of self-regulation, impossible to match a correcting variation of the speed of one motion to appropriate variables of the other motion.
The attempts to attain an equality between the speed of bringing the workpieces together and the speed of their flashing have failed, inasmuch as the inertia of the mechanical units of the continuous flash butt-welding machine (in the embodiment of the invention under consideration, of the column and its drive unit) and of the electrical equipment controlling the translatory motion resulted in delays and linear inaccuracies in execution of control signals which were commensurable with, and even exceeded, the time and linear parameters of the oscillatory motion and thus nullified the effect of the oscillatory motion.